A lighting circuit for a discharge lamp, such as a metal halide lamp, is known which incorporates a DC power source, a switching power source circuit, a DC-AC converting circuit and a start circuit. The switching power source circuit is formed into a DC-DC converter circuit using a DC chopper type or a flyback type circuit.
The DC-AC converting circuit for converting DC voltage into AC voltage (rectangular waves or the like) uses a known structure of a full bridge circuit having a plurality of pairs of switching devices (or switch devices). The structure causes two pairs of switching devices to be operated contradictorily to obtain alternating output.
FIG. 5 shows an equivalent circuit of an example of a conventional circuit structure. A lighting circuit 31 comprises a switching power source portion including a DC power source 32, an inductor 33, a switching device 34 (for example, a FET (Field Effect Transistor) or the like), and a full bridge circuit 36 (operating circuits for the devices are omitted from illustration) including four switching devices 35-1 to 35-4 (each of which is indicated with the symbol of a switch).
That is, the positive electrode of the DC power source 32 is connected to an end of the inductor 33. Another end of the inductor 33 is connected to the drain of an N-channel FET which is the switching device 34. The output voltage from the switching power source portion is supplied to the full bridge circuit 36. Note that the source of the FET is connected to a negative electrode of the DC power source 32. A signal transmitted from a control circuit (not shown) is supplied to the gate of the FET.
The switching devices constituting the full bridge circuit 36 are disposed in the higher and lower stages. The switching devices 35-1 and 35-3 are disposed in the upper stage, while the switching devices 35-2 and 35-4 are disposed in the lower stage. An end of the switching device 35-1 is connected to the inductor 33, while another end of the switching device 35-1 is connected to the switching device 35-2. An end of the switching device 35-3 is connected to the inductor 33, while another end of the switching device 35-3 is connected to the switching device 35-4. Note that a discharge lamp g has an end connected to a connection point A between the switching devices 35-1 and 35-2. Another end of the discharge lamp 37 is connected to a connection point B between the switching devices 35-3 and 35-4.
A portion of the terminals of the switching devices 35-2 and 35-4 disposed opposite to the connection point of the discharge lamp 37 are connected to the source of the FET through a shunt resistance 38 provided to detect an electric current of the discharge lamp 37.
In the lighting circuit 31, the output voltage from the switching power supply portion is controlled in response to a control signal supplied to the switching device 34 so as to be supplied to the full bridge circuit 36. In the full bridge circuit 36, combination of the switching devices 35-1 and 35-4 and that of switching devices 35-2 and 35-3 are constituted so as to contradictorily be switched. Thus, a rectangular AC output is supplied from each of the connection points A and B to the discharge lamp 37.
There is a great danger of the conventional circuit producing anomalous heat, smoke or fire if breakdown of the switching device occurs due to a short circuit owing to a passing current generated when either electrode of the discharge lamp encounters a ground fault.
A case will now be described in which a lighting circuit of a discharge lamp for a vehicle encounters a short circuit between either electrode of the discharge lamp and the car body. If short circuit occurs in an output stage of the full bridge circuit 36 as indicated with a two-dot chain line shown in FIG. 5, a passing current I undesirably flows through a route as indicated with a dashed line shown in FIG. 5 in a state where the switching devices 35-1 and 35-3 are switched on and the switching devices 35-2 and 35-4 are switched off.
The passing current I causes wasteful consumption of electric currents of the DC power source 32. What is worse, heat and fire can be produced by the switching devices 35-1 to 35-4 of the full bridge circuit 36. Therefore, the passing current I must be prevented.
To prevent generation of the passing current I, the passage for the electric current must be obstructed at the switching devices 35-1 and 35-3 in the higher stage. Therefore, it might easily be considered to employ a method of switching the switching devices 35-1 and 35-3 off.
In a state shown in FIG. 6, an occurrence of the passing current I can be prevented if the switching devices 35-1 and 35-3 in the higher stage are switched off and the switching devices 35-2 and 35-4 in the lower stage are switched on, and the operations of the switching devices 35-1 and 35-3 are always normal.
However, if the switching device 35-3 is ruptured because of a short circuit generated due to some reason, passing current I' undesirably flows through a route (the positive electrode of the DC power source 32 to the switching device 35-3 to the switching device 35-4 to the shunt resistance 36 to the negative electrode of the DC power source 32) indicated with a dashed line shown in FIG. 6.
To prevent generation of the passing currents I and I', a method must be employed to switch all of the switching devices 35-1 to 35-4 off. The conventional structure and control of such an operating circuit (a so-called "bridge driver) for the switching devices required to realize this state become too complicated. It leads to a fact that the cost is raised excessively. Also in the foregoing case, all of the switching devices 35-1 to 35-4 can be switched off only when each of the switching device is free of any anomaly.